Peanut worms (Sipuncula) have been variously related to a number of animals in the past, but, recently, and despite their non-segmented adult body, molecular phylogenetic analyses place them within Annelida. In order to contribute to the question whether or not sipunculan worms show traces of a segmental pattern in their ontogeny, neuro- and myogenesis as well as the distribution of proliferating cells were analysed in three different sipunculan species. The data show that the rudiments of numerous circular muscles of the body wall musculature and the longitudinal retractor muscles appear at the same time in the early trochophore larva. In addition, throughout development newly formed ring muscles emerge along the entire anteriorposterior axis by fission from already existing myocytes. In contrast to that, neurogenesis does follow a segmental pattern by subsequently emerging pairs of serotonergic perikarya along the anterior-posterior axis, which are associated with a paired ventral nerve cord and are arranged in four distinct repetitive units. This metameric pattern, however, disappears prior to metamorphosis and the ventral nerve cords fuse to form the single ventral nerve of the adult. Congruently, the distribution pattern of mitotic cells show similarities to an annelid-like posterior growth zone that also disappears in metamorphic competent pelagosphera larvae. Accordingly, these developmental and morphological data corroborate recent molecular analyses and show that sipunculans stem from a segmented ancestor. Furthermore, the loss of segmentation in these relatively large, free living animals indicates that body segmentation may easier be lost during evolution than previously assumed. The ontogenetic establishment and (secondary) loss of segmentation renders Sipuncula ideal for developmental and evolutionary studies on the segmentation process in Metazoa.